This prefix resolver is created whenever null is passed to the
evaluate method. Its purpose is to satisfy the DOM L3 XPath API
requirement that if a null prefix resolver is used, an exception
should only be thrown when an attempt is made to resolve a prefix.

getBaseIdentifier

The base identifier from where relative URIs should be absolutized, or null
if the base ID is unknown.

CAVEAT: Note that the base URI in an XML document may vary with where
you are in the document, if part of the doc's contents were brought in
via an external entity reference or if mechanisms such as xml:base have
been used. Unless this PrefixResolver is bound to a specific portion of
the document, or has been kept up to date via some other mechanism, it
may not accurately reflect that context information.